1. Field of the Invention
The present invention relates to a fixed subscriber unit in which at least one telephone set connected thereto by wire can send and receive calls by radio to and from and speak with another telephone set connected to a public telephone network, or more particularly to a fixed subscriber unit in which the user is supplied with the information as to whether the fixed subscriber unit is driven by an AC/DC adaptor or by a power back-up charge battery unit and so on.
2. Description of the Related Art
FIG. 1 is a block diagram showing a configuration of a conventional fixed subscriber unit. A conventional fixed subscriber unit 1 is connected by wire to one or a plurality of telephone sets for sending and receiving through an antenna 2 a radio wave to and from a base station (not shown) connected to a public telephone network. A radio section 3 demodulates the signal received from the antenna 2 into an audio signal on the one hand and modulates the audio signal from the telephone 13 and transmits it by way of the antenna 2 on the other hand. A two-wire/four-wire converter 4 converts two telephone wires for connecting the telephone set 13 to a total of four wires including two transmit wires and two receive wires to be connected to the radio section 3. In the case of a non-balanced circuit, one of the two transmit or receive wires is connected to the earth and is not shown.
A ringing tone generator 6 sends out a ringing tone for sounding the ringer of the telephone set 13. A relay 7 switches the telephone wires for connection to the telephone 13 between the two-wire/four-wire converter 4 and the ringing tone generator 6. A hooking detector 8 is for detecting the on/off-hook state of the telephone set 13 by monitoring the current in the telephone wire connected to the telephone set 13. A DTMF detector 9 detects a DTMF signal sent by the telephone set 13. A DT.multidot.BT generator 10 generates a DT (dial tone) and BT (busy tone) to be sent to the telephone set 13.
A controller 21 controls the radio section 3, the ringing tone generator 6, the relay 7 and the DT.multidot.BT generator 10 by the signals from the radio section 3, the hooking detector 8 and the DTMF detector 9, so that the telephone set 13 can send and receive a call and speak by radio with other telephone sets connected to the public telephone network. The telephone set 13 is connected by wire to the fixed subscriber unit 1.
A power circuit 12 supplies a DC voltage to each part in the fixed subscriber unit 1. An AC receptacle cable 14 transmits an AC voltage from a home AC receptacle to an AC/DC adaptor 15. The AD/DC adaptor 15 converts an AC voltage to a predetermined DC voltage and outputs the DC voltage.
A power back-up charge battery unit 16 supplies a DC voltage, when supplied from the AC/DC adaptor 15, directly to the power circuit 12 while at the same time charging the charge battery thereof, if the battery is not sufficiently charged. In the case where the DC voltage is not supplied from the AC/DC adaptor 15 due to a power failure or the like cause, the DC voltage of the charge battery is supplied to the power circuit 12. This power back-up charge battery unit 16 can be either built in or attached to the exterior of the fixed subscriber unit 1.
Now, the operation of the conventional fixed subscriber unit will be described. In FIG. 1, upon receipt of a call from another telephone through a radio base station, the controller 21 receives call-in information from the radio section 3. Then, the controller 21 controls the ringing tone generator 6 and the relay 7 to send out a ringing tone to the telephone set 13. When the telephone 13 goes off the hook, the controller 21 acquires the off-hook information from the hooking detector 8, controls the relay 7 and connects the telephone 13 with the other telephone for transmitting and receiving signals through the antenna 2, the radio section 3 and the two-wire/four-wire converter 4.
In this way, the conventional fixed subscriber unit 1 makes possible speech by radio between at least one telephone set connected by wire to the fixed subscriber unit 1 and other telephone sets connected to the public telephone network.
As to the power system, the AC voltage obtained from a home AC receptacle is converted into a fixed DC voltage by the AC/DC adaptor 15, and supplied to the power circuit 12 through the power back-up charge battery unit 16. In the process, the power back-up charge battery unit 16, if the charge battery thereof is not sufficiently charged, supplies the DC voltage of the AC/DC adaptor 15 to the power circuit 12 while at the same time charging the charge battery thereof. In the case where the DC voltage output of the AC/DC adaptor is stopped due to a power failure or a loosened AC receptacle, the DC voltage output of the charge battery in the power back-up charge battery unit 16 is supplied to the power circuit 12, so that the operation of the fixed subscriber unit 1 is made possible for a time period during which the DC output voltage of the charge battery is maintained in a certain range.
In the above-mentioned fixed subscriber unit 1 described above, the user cannot not be informed in the case where the DC voltage output of the AC/DC adaptor 15 stops due to a power failure or a loosened AC receptacle cable 14 and the fixed subscriber unit 1 is driven by the output voltage of the charge battery of the power back-up charge battery unit 16. The user is, therefore, neither informed that the remaining available speech time is limited since the power for the operation is supplied from the power back-up charge battery unit 16.